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Lattuada M.,Institute for Chemical and Bioengineering
Journal of Physical Chemistry B

Smoluchowski's equation for the rate of aggregation of colloidal particles under diffusion-limited conditions has set the basis for the interpretation of kinetics of aggregation phenomena. Nevertheless, its use is limited to sufficiently dilute conditions. In this work we propose a correction to Smoluchowski's equation by using a result derived by Richards (J. Phys. Chem. 1986, 85, 3520) within the framework of trapping theory. This corrected aggregation kernel, which accounts for concentration dependence effects, has been implemented in a population-balance equations scheme and used to model the aggregation kinetics of colloidal particles undergoing diffusion-limited aggregation under concentrated conditions (up to a particle volume fraction of 30%). The predictions of population balance calculations have been validated by means of Brownian dynamic simulations. It was found that the corrected kernel can very well reproduce the results from Brownian dynamic simulations for all concentration values investigated, and is also able to accurately predict the time required by a suspension to reach the gel point. On the other hand, classical Smoluchowski's theory substantially underpredicts the rate of aggregation as well as the onset of gelation, with deviations becoming progressively more severe as the particle volume fraction increases. © 2011 American Chemical Society. Source

Singh J.,Institute for Chemical and Bioengineering | Lamberti C.,University of Turin | Van Bokhoven J.A.,Institute for Chemical and Bioengineering | Van Bokhoven J.A.,Paul Scherrer Institute
Chemical Society Reviews

Knowledge of the structure of catalysts is essential to understand their behavior, which further facilitates development of an active, selective, and stable catalyst. Determining the structure of a functioning catalyst is essential in this regard. The structure of a catalyst is prone to change during the catalytic process and needs to be determined in its working conditions. In this tutorial review, we have summarized studies done at synchrotron radiation facilities that illustrate the capability to determine catalyst structure using X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). These studies aim at facilitating the determination of the dynamic structure-performance relationships during a catalytic process. © 2010 The Royal Society of Chemistry. Source

Furlan M.,Institute for Chemical and Bioengineering | Lattuada M.,Institute for Chemical and Bioengineering

Sol-gel accompanied by phase separation is an established method for the preparation of porous silica monoliths with well-defined macroporosity, which find numerous applications. In this work, we demonstrate how the addition of (superpara)magnetic nanocolloids as templates to a system undergoing a sol-gel transition with phase separation leads to the creation of monoliths with a strongly anisotropic structure. It is known that magnetic nanocolloids respond to the application of an external magnetic field by selfassembling into columnar structures. The application of a magnetic field during the chemically driven spinodal decomposition induced by the sol-gel transition allows one to break the symmetry of the system and promote the growth of elongated needlelike silica domains incorporating the magnetic nanocolloids, aligned in the direction of the field. It is found that this microstructure imparts a strong mechanical anisotropy to the materials, with a ratio between the Young's modulus values measured in a direction parallel and perpendicular to the one of the field as high as 150, and an overall smaller average macropores size as compared to isotropic monoliths. The microstructure and properties of the porous monoliths can be controlled by changing both the system composition and the strength of the applied magnetic field. Our monoliths represent the first example of materials prepared by magnetically controlling a phase transition occurring via spinodal decomposition. © 2012 American Chemical Society. Source

Andanson J.-M.,Institute for Chemical and Bioengineering | Baiker A.,Institute for Chemical and Bioengineering
Chemical Society Reviews

In situ attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy has gained considerable attention as a powerful tool for exploring processes occurring at solid/liquid and solid/liquid/gas interfaces as encountered in heterogeneous catalysis and electrochemistry. Understanding of the molecular interactions occurring at the surface of a catalyst is not only of fundamental interest but constitutes the basis for a rational design of heterogeneous catalytic systems. Infrared spectroscopy has the exceptional advantage to provide information about structure and environment of molecules. In the last decade, in situ ATR-FT-IR has been developed rapidly and successfully applied for unraveling processes occurring at solid/liquid interfaces. Additionally, the kinetics of complex reactions can be followed by quantifying the concentration of products and reactants simultaneously in a non-destructive way. In this tutorial review we discuss some key aspects which have to be taken into account for successful application of in situ ATR-FT-IR to examine solid/liquid catalytic interfaces, including different experimental aspects concerned with the internal reflection element, catalyst deposition, cell design, and advanced experimental methods and spectrum analysis. Some of these aspects are illustrated using recent examples from our research. Finally, the potential and some limitations of ATR will be elucidated. © 2010 The Royal Society of Chemistry. Source

Amrute A.P.,Institute for Chemical and Bioengineering | Mondelli C.,Institute for Chemical and Bioengineering | Hevia M.A.G.,Institute of Chemical Research of Catalonia | Perez-Ramirez J.,Institute for Chemical and Bioengineering
Journal of Physical Chemistry C

The use of in situ approaches to study mechanisms of experimentally demanding and complex reactions under conditions of practical relevance opens doorways to discover new or improved heterogeneous catalysts. A representative example within this category of reactions is the gas-phase oxidation of HCl to Cl2 (Deacon process). Studies in the temporal analysis of products (TAP) reactor complemented by flow experiments at ambient pressure evidenced pronounced mechanistic differences between copper catalysts (CuO, CuCl, and CuCl2, that is, Deacon-like catalysts) and RuO2 (the basis of the recent Sumitomos catalyst for large-scale Cl2 production). RuO2 is 1 order of magnitude more active than the copper-based materials. HCl oxidation on RuO2 obeys a Langmuir-Hinshelwood mechanism in the presence of adsorbed species over a partially chlorinated surface. HCl oxidation on the copper catalysts is more complex: all the samples experience bulk changes, namely, chlorination, resulting in multiphase materials. Besides, lattice species also participate in the reaction. In particular, fresh CuO follows a Mars-van Krevelen mechanism, in which lattice oxygen is active for Cl2 and H2O production, leading to bulk chlorination, whereas for copper chlorides activated upon exposure to the Deacon mixture and used CuO, a combination of Mars-van Krevelen and Langmuir-Hinshelwood mechanisms seems to be a better description. Investigations over the copper-based phases and characterization of the fresh and used samples by X-ray diffraction indicate that a copper (hydr)oxychloride is the main active phase. Our mechanistic study suggests that a more active and particularly stable copper catalyst can be achieved by controlling the degree of surface chlorination. This is more or less a self-regulated process on the ruthenium-based catalyst. © 2010 American Chemical Society. Source

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